1. Field of the Invention
The present invention relates generally to a memory cell structure for semiconductors, and more particularly, to chalcogenide memory cells.
2. Description of Related Art
Integrated circuits, typically in the form of microprocessors, microcontrollers, or other logic circuits, are used to control the functions of many modern electronic devices. For example, integrated circuits are used to control the functions of computers, telephones, and many other consumer electronics. It is generally necessary for the integrated circuits to retrieve (read) and store (write) data as they perform their functions. The data may be in the form of instructions for the integrated circuits (e.g., a program), data necessary for the execution of a program, or data generated during the execution of the program. It is highly advantages to store the data in memory devices which are easily accessible by the integrated circuits.
Many different types of memory devices are known for the storage of data. In selecting a memory device, one should keep in mind the particular requirements for the data with which the memory device will be used. For example, the quantity of data and the required access time to the data can play an influential role in memory device selection. Regarding high-capacity memory devices, floppy disk drives, hard disk drives, compact disks (CDs) and digital video disks (DVDs) are commonly used to store large amounts of data and/or large programs. While facilitating dense storage of data/programs in a nonvolatile format, these memory devices require motors and other electro-mechanical devices to operate. Accordingly, relatively large amounts of electricity can be consumed and access time to the data can be attenuated.
Integrated circuits may also be used to form memory devices. Two common types of integrated circuit memory devices are random access memory (RAM) and read only memory (ROM). Many improvements and variations of RAMs and ROMs have been advanced to further their performance, a substantial percentage of which improvements are commercially available. Memory devices constructed of integrated circuits are relatively small, durable, and consume small amounts of electricity making them very adaptable and easy to use. In addition, integrated circuit memory devices typically have faster access, reading and writing times, compared to other types of memory devices.
However, conventional RAMs and ROMs have their own disadvantages. RAMs, while having fast data transfer rates and efficient writing architectures, must be continuously powered or their memory contents may be compromised. Thus, RAMs may loose their data, i.e. they are volatile, when power is inadvertently or accidentally removed. ROMs, while retaining their contents even in the absence of power, are limited in the number of times to which they may be written, the speed, and the manner in which they may be written. RAMs and ROMs, even with their limitations, find overwhelmingly practical applications in a majority of electronic devices. However, a need exists in the prior art for memory devices that can harness the advantages of integrated circuits while avoiding the disadvantages of RAMs and ROMs.
In an effort to create integrated circuit memory devices which are nonvolatile and which may be quickly and repeatedly written to, phase changing materials have been researched. Specifically, chalcogenide materials have been studied for their use as storage media for data. Chalcogenide materials generally comprise one or more of the elements Ni, Se, Te, Sb and Ge and are considered as being able to change phases, or states, while being incorporated into integrated circuits. Specifically, chalcogenide materials may be switched from a generally amorphous state to a generally crystalline state, or to an intermediate state there between, through the application of a switching current. Chalcogenide materials have greater resistances in the amorphous state than they have in the crystalline state. Thus, the resistance of the chalcogenide material may be set to represent or encode specific data to be stored. Once the data has been set in the form of a varying resistance level, resistances of the chalcogenide material may later be measured, without altering the data, in order to read the data that was stored.
Regarding the storage of binary code, chalcogenide materials may be configured to store either high or low resistive states, corresponding to logic “1” and logic “0” values, respectively. In addition, they may be used to store analog information by storing a resistive state along a spectrum of continuous resistive states. Chalcogenide materials are also fast and consume very little electrical current. Specifically, they are capable of being repeatedly switched between different resistive states within nanoseconds while consuming only picojoules of electrical energy. Another advantage of chalcogenide materials is that they are nonvolatile, being capable of retaining their resistive state for extended periods of time without the need for continuous power.
Methods of making and using chalcogenide materials to form chalcogenide memory cells are disclosed in U.S. Pat. Nos. 5,687,112 to Ovshinsky; 5,789,277 to Zahorik et al.; 5,837,564 to Sandhu et al.; 5,879,955 to Gonzalez et al.; 6,031,287 to Harshfield; 6,104,038 to Gonzalez et al.; 6,111,264 to Wolstenholme et al.; 6,147,395 to Gilgen; and 6,150,253 to Doan et al., all of which are hereby incorporated by reference in their entireties. As set fourth in the prior art, a typical chalcogenide memory cell will comprise a storage region which may be adjusted between an amorphous and a crystalline phase state. A current pulse, of only a few picojoules of energy having a current density between about 105 and 107 amperes per square centimeter, may be used to set the phase state of the storage region. This electrical current, which can become quite significant when millions or more memory cells are being used, may be reduced with the creation of smaller storage regions. In addition, smaller storage regions would allow for a greater density of memory cells to be created, thereby enhancing the commercial value of chalcogenide memory cells.
While advantages of smaller storage regions for chalcogenide memory cells are suspected, improved designs and manufacturing techniques are still needed to further reduce their size. The present invention is directed to overcoming, or at least reducing the affects of, one or more of the problems set forth above.